The present invention relates to data networks, to nodes for such networks, to methods of transmitting data, and to corresponding software. More particularly, it relates to data transmission networks having working paths and secondary paths for use by working traffic when there is a fault on the working path.
A number of mechanisms are known for handling fault conditions in data transmission networks or systems. At higher levels of the well known OSI hierarchy, packets may be buffered and resent. At lower levels of the OSI hierarchy, if there is control over the route taken by the data, then an alternative route through the network can be tried. An example of an OSI layer 1 protocol, is SONET, and its equivalent outside North America, SDH. Such layer 1 transmission systems typically have secondary (usually termed protection) paths arranged such that working traffic is switched on to these secondary paths in the event of failure at a node or on a line between nodes of the system.
Typically, the nodes and lines are arranged to form rings. The protection paths are typically arranged either in the form of dedicated path protection rings (DPRing) or shared protection switched rings (SPRing). There are two kinds of SPRing, unidirectional line switched rings, or bi-directional line switched rings (BLSR). Bell Core Standard GR-1230-CORE Issue 2, defines BLSR for SONET. The standard includes protocols for extra traffic to use the protection paths when they are not in use by the working traffic. Paths are defined here as extending between neighbouring nodes, so typically data will pass over several such paths to reach its destination. Many paths can be multiplexed onto a single span, e.g. by time division multiplexing according to SONET/SDH standards, or by wavelength division multiplexing for example.
Where there is a ring configuration, and a fault is detected on a line on the working path, there are two possibilities in principle, for re-routing the working traffic to avoid the faulty line. Firstly, it could be switched on to the protection path on the same span as the faulty working path. This is known as a span switch. Secondly, the working traffic could be routed on the protection path the other way around the ring, using spans other than the faulty span. This is called a ring switch. In general, span switching is preferred since it occupies fewer of the protection paths available on the ring. This means several span switches may occur on the same ring, but only one ring switch.
There has long been a concern about the bandwidth efficiency of such systems, if 50% of the total bandwidth is reserved for the protection paths. Many efforts have been made to improve the efficiency. For example, extra traffic is carried on the protection channels. This extra traffic differs from working traffic in the treatment accorded to it at higher levels of the well known OSI hierarchy, such as priority level, charging rate, monitoring and so on. At the transport layer, it is treated the same as other traffic except for being given a lower priority under fault conditions, so it is unprotected and is automatically removed if a protection request is made, to switch the working traffic on to the protection path. In practice, it proves hard to derive much revenue from unprotected traffic.
Other attempts to improve bandwidth efficiency have involved efforts to share the protection path between several working paths. For example, where neighbouring rings overlap, a single protection path can serve both rings.
If the interconnections between nodes are arranged in a mesh rather than in one or more rings, then there may be many routes between any pair of nodes. In this case, in the event of a fault on one span, the traffic can be divided and sent over the protection paths for the various alternative routes to the destination node. Thus each protection path can have much less than 50% of the bandwidth of a given span between nodes, and the overall bandwidth efficiency can be much greater than 50% and still guarantee protection against any single fault. Again extra traffic can be sent over the protection paths. Compared to ring arrangements, the main drawback of mesh networks is the complexity of managing the network and deciding which protection paths to use in the event of a fault. Even if this is pre-calculated, changing traffic patterns may warrant complex re-configurations.
Yet other attempts at improving efficiency have involved removing completely unused working paths.
In summary, network operators perceive rings to be inefficient, and meshes to be efficient, yet difficult to manage and operate. Extra traffic to make use of the idle protection bandwidth is seldom used, because span switches occur often enough to cause too many outages on the extra traffic.
It is an object of the invention to provide improved arrangements which address the above mentioned problems.
According to the invention there is provided a communication network comprising a plurality of nodes linked by spans, to carry working paths between the nodes for use by working traffic and to carry protection paths, the nodes being arranged to use the protection paths for extra traffic, when the protection paths are not being used for working traffic, the nodes being arranged to use one or more of the protection paths for working traffic in the event of a fault on one of the working paths, and thus displace extra traffic from the protection path or paths used by the working traffic, the nodes further being arranged to use an alternative path to protect at least some of the displaced extra traffic.
Having at least some of the extra traffic protected rather than being discarded, in the event of a fault, can enable better bandwidth utilisation, regardless of how the working traffic is protected. Furthermore this way of achieving better bandwidth utilisation may be simpler to manage and operate compared to trying to achieve a corresponding bandwidth utilisation improvement in other known ways such as by providing more meshed paths for a mesh restoration of the working traffic. While this known method has the disadvantage of introducing more complexity into managing the switching decisions for the mesh restoration, the invention can be combined with such known methods or be regarded as an alternative.
Preferably the alternative path comprises at least some of the protection paths not occupied by working traffic. This can be simpler and easier to manage and operate because these paths may be of the same type, same level of service and so on. Also this can improve levels of bandwidth utilisation, since more protected traffic can be accommodated.
Preferably at least some of the protection paths form a protection ring and the alternative path comprises the remainder of the protection ring to the part normally used by the respective extra traffic. This use of the other way around the protection ring provides a guaranteed alternative path and is easier to manage and operate than a mesh arrangement of protection paths. Each node need be aware only of the node in its own ring, not of the entire network. Rings inherently provide predetermined routes between nodes, making management and operation simpler and faster.
Preferably the nodes are arranged such that there is a predetermined configuration of which of the protection paths are used to protect respective ones of the working paths. This enables protection to occur more quickly.
Preferably the nodes are arranged to send the extra traffic simultaneously both ways around the protection ring. This simplifies the protection of the extra traffic because under single fault conditions, the extra traffic is already present on an alternative path and so the need for switching is avoided and the protection can occur more quickly. UPSR (unidirectional path switched ring) is an example of this.
Preferably the alternative path is formed by providing path loop back either side of the fault, the nodes further being arranged to pass this extra traffic around the protection ring to its destination. This is simpler than head end ring switching because only the nodes which provide the loop back need to change to create the alternative path. For head end ring switching, the nodes where the extra traffic enters the ring must change which direction they send the extra traffic around the ring. This is more complex because many nodes must change. However the loop back scheme may use more bandwidth than head end ring switching because any traffic using the loop back will pass twice over the same span. This will be relatively easy to manage, and fast to configure, because each node needs only to decide whether it is a loop back node or an intermediate node. Then in operation, each node simply identifies whether incoming traffic on the protection ring should be dropped or passed through. There is no need for each node to be aware of the arrangement of other nodes on the ring.
Preferably the above mentioned bi-directional rings for the working paths are combined with the above mentioned arrangement of protection paths in the form of a ring. This enables the advantages of rings to be obtained for both levels of protection, that is protection of the working traffic, and protection of at least some of the extra traffic.
Preferably the ring for the working traffic comprises a four fibre BLSR, and the ring for the protection path is in the form of a two fibre BLSR. This enables the invention to be applied to the large installed base of the four fibre BLSRs, and obtain the advantages of both types of BLSR. More specifically, well defined protocols already exist for both types of BLSR and therefore implementation will be easier.
Preferably the working paths are linear and the protection paths form a ring. For example 1:n, 1:1 or m:n arrangements (where n and m are positive integers) are conceivable. If the protection paths are shared between many working paths in a 1:n arrangement, a priority scheme will be used to determine which of the working paths is protected. This is essentially independent of and complementary to the priority scheme for the extra traffic described above.
Preferably, the extra traffic is pre-configured into two levels of priority, the higher priority extra traffic being protected, the lower priority extra traffic being discarded in the event of a fault. By predetermining the priority of the extra traffic, the higher priority extra traffic can be guaranteed to a higher level of reliability.
Preferably the protection of the working traffic or of the displaced extra traffic is carried out according to signalling in SONET/SDH overhead.
Preferably the protection paths are arranged in a mesh. This may enable more of the extra traffic to be protected so the efficiency can be improved, though at the expense of more complexity.
Preferably the re-routing comprises head end switching. This involves determining which direction to route traffic around the ring, at the point of adding the traffic to the ring, and enables the shortest path to be taken, to avoid the need for a loop back. This involves more complex configuration but may enable better use of bandwidth on a ring. This is more efficient than loop back or UPSR since it takes the shortest route. However it may be more complex to manage and operate because each source and destination node must know enough to send/receive traffic to/from the shortest path.
Preferably the nodes are arranged to time division multiplex working traffic from different sources over the same working path.
Preferably the switching of the working traffic is carried out in the optical domain.
Preferably the nodes are arranged to wavelength division multiplex working traffic from different sources onto the same working path, and wavelength division multiplex the protection and working paths on a respective span.
According to another aspect of the invention there is provided a data transmission network comprising a plurality of nodes, an arrangement of high priority paths between the nodes, for carrying high priority traffic and a plurality of lower priority paths between the nodes, for use by the high priority traffic in the event of a fault in one of the high priority paths, and for use otherwise by lower priority traffic, the lower priority paths forming a ring, the nodes being arranged to carry out a span switch operation to replace a respective one or more of the high priority paths by a corresponding one or more of the lower priority paths, the nodes further being arranged to carry out a ring switch to send at least some of the lower priority traffic around the ring of lower priority paths, and avoid those of the low priority paths used by the span switch operation.
The combination of a span switch to protect data traffic on the high priority paths and a ring switch to protect the lower priority traffic can enable the lower priority traffic availability to be enhanced in a way which makes better use of the lower priority paths without affecting the high priority paths. Also, span switching and ring switching can be implemented using well established techniques, and so the advantages can be obtained with less risk and cost than other techniques. Furthermore, span and ring switching can be more rapid than mesh schemes for example, since intermediate nodes need not be reconfigured, and there is no need to spend time calculating which is the optimum route.
The high priority paths may be working paths, the low priority paths may be protection paths, and the lower priority traffic may be extra traffic for example, though clearly the invention is applicable beyond these examples.
According to other aspects of the invention, there are provided a node for use in the above network, a method of operating a network, a method of operating the node, and corresponding software. Any of the preferred features may be combined with any of the aspects set out above as would be apparent to a skilled person. Other advantages will be apparent to a skilled person, particularly in relation to any further prior art other than that discussed above.